An aircraft, and the gas turbine engines that power it, usually include numerous units that require electrical, hydraulic or pneumatic power in order to function. Conventionally, the power for such units is derived from one of three sources depending upon the operating status of the aircraft. If the aircraft is stationary and it is necessary for certain electrical support systems to be operational, a ground-based electrical generator is coupled to the aircraft. Alternatively, the electrical power may be derived from an aircraft mounted generator which is powered by an auxiliary power unit mounted on the aircraft. Such an auxiliary power unit is commonly in the form of a small gas turbine engine mounted in the tail region of the aircraft.
If the aircraft is airborne, power for the units is derived from the engines which power the aircraft. Typically, each engine is provided with an auxiliary shaft that transmits power from one of the main shafts of the engine to a gearbox mounted on the engine external casing. The gearbox in turn drives various electrical generators and hydraulic pumps. A single electrical generator may be driven by the gearbox. However in the event of the failure of that generator, there would, of course, be an interruption in the supply of electrical power to the engine and aircraft. In a multi-engined aircraft such an interruption may be acceptable for a short time period since electrical power is still likely to be available from the remaining engine or engines. However, if the aircraft is required to fly in regions in which a diversionary airfield is some distance away, it is usually necessary for the gearbox to drive an additional electrical generator for back-up purposes.
Such duplication of electrical power generators leads to difficulties in conveniently positioning them relative to the gearbox and also to complex and heavy modifications to the gearbox to enable it to drive both generators.